Natasha Romanoff

Trained as an assassin from childhood in a programme that forcibly sterilised its recruits and chemically conditioned them so they physically could not disobey. She escaped and defected via Clint Barton, who was sent to kill her and chose not to.

She recruits Banner, gets Loki's plan out of him by letting him think he has won, and closes the portal at the end.

Exposing HYDRA means exposing S.H.I.E.L.D., because they are the same organism — which means burning every operation she has ever run. She does it in front of a Senate committee anyway. The cost is that every intelligence agency on Earth now holds those files.

Rogers and Romanoff train a new roster: Wilson, Rhodes, Wanda, Vision.

Team Stark: Stark, Rhodes (paralysed in the fight), Vision, Romanoff (who defects mid-battle), T'Challa, and a recruited 15-year-old Peter Parker.

Set immediately after Civil War, while Natasha is a fugitive. A "Black Widow" is the product of a programme. Natasha escaped and defected via Clint Barton, who was sent to kill her and chose not to.

A fugitive alongside Rogers and Wilson. She is in the Edinburgh rescue and on the line at Wakanda.

She and Clint Barton fight over which of them will die for the Soul Stone. She wins. The film never addresses how you return a Stone that cost a life, and nothing since has either — Rogers goes back to Vormir, and she does not come back.